Ruzzelli, Cotan O’Hare, Tynan, Havinga Protocol assessment issues in low duty cycle sensor networks: The switching energy A.G. Ruzzelli, P. Cotan*, G.M.P. O’Hare, R. Tynan, and P.J.M Havinga** Adaptive Information Cluster (AIC) PRISM Laboratory School of Computer Science and Informatics, University College Dublin (UCD), Ireland. *Department of Electronic Engineering, Technical University of Catalonia, Spain. **Department of Computer Science, University of Twente, The Netherlands.

Ruzzelli, Cotan O’Hare, Tynan, Havinga Summary Generality on protocol energy assessment The low duty Cycle through the wake up concept Switching between transceiver states Phase1: Measurements on board –The sensor node –The experimental approach –The measured results Phase2: Switching energy assessment –The S-MAC protocol –Performance evaluation –Simulation setup –Simulated results Considerations Conclusions

Ruzzelli, Cotan O’Hare, Tynan, Havinga Generality on protocol energy assessment Energy consumption is mainly due to the transceiver activity; Protocol energy assessment is based on transceiver time states: –Transmit time; –Receive time; –Idle time (Sleeping time in sensor networks); –Switching time (USUALLY NOT ASSESSED); Energy spent in state switching is negligible in ad-hoc wireless network protocol assessment (e.g. WiFi);

Ruzzelli, Cotan O’Hare, Tynan, Havinga Switching in standard wireless networks Is defined as the transition time that elapses between the end of a transceiver state and the beginning of the following one; Possible switching states consist of: –RX/TX and TX/RX switching states –TX/Sleep and Sleep/TX switching states –RX/Sleep and Sleep/RX switching states State transition is fast hence very little amount of energy is consumed. Switching energy is much smaller than the energy spent in other transceiver states. Transceiver data sheets report average switching time but not the energy spend. Related work show that assessment of novel protocol architectures for WSNs inherited the switching energy negligibility.

Ruzzelli, Cotan O’Hare, Tynan, Havinga Sensor network characteristics Energy consumption is primary objective Introducing the wake-up concept Very low duty cycle (even less than 5%) Packets are much smaller than in common ad-hoc networks (e.g. temperature data is few bytes) Little amount of data transmitted per node Can we consider switching energy still negligible for low duty cycle sensor networks?

Ruzzelli, Cotan O’Hare, Tynan, Havinga Phase 1: The experimental model Analysis conducted on different EYES sensor node prototypes Each prototype mounted a different off-the shelf transceiver for sensor networks; Tr1001, CC1000 and CC1010 transceivers have been investigated;

Ruzzelli, Cotan O’Hare, Tynan, Havinga Phase 1:The experimental approach The voltage drop is gauged across a known high-side series resistor placed between the battery (+ terminal) and the sensor node input power connector; Current consumption, power and energy consumption could be derived from the voltage. The hardware has been connected to an oscilloscope then repetitive cycle of node switching between states has been performed.

Ruzzelli, Cotan O’Hare, Tynan, Havinga The measuring circuit Based on the INA110 instrumentation amplifier fast settling time and high slew rate device. Two resistors for “low power mode” and “Tx/Rx mode” current ranges has been used. Test performed by a square waveform of 1 kHz and of 5 V amplitude at the input of the INA 110 connected through an attenuating resistive divider circuit. The circuit offered good precision allowing us to conduct measurements at the edges with a very low distortion. INA110 main characteristics Bias50 pA max Settling time (Vout 20V)3 us to 0.1 % CMRR106 dB min Gain1, 10, 100, 200, 500 Input impedance5x10^12 ohm || 6pF Slew Rate17 V/us Small signal BW470 kHz (Gain = 100)

Ruzzelli, Cotan O’Hare, Tynan, Havinga Preliminary notes: The CC1010 had a processor built in and therefore the CPU on that board could be put into sleep mode. CC chipcon class presented higher sensitivity than TR1001. CC1000 and CC1010 boards were configured with the oscillator ON in low power mode  shorter switching time (2ms activation if OFF) Board measurement results Boards current and power consumption Current [mA]Power [mW] SLRXTXSLRXTX TR CC CC Boards switching energy Boards switching times Switching Energy [uJ] SL to RXSL to TXRX to SLTX to SLRX to TXTX to RX TR CC CC Switching Times [us] SL to RXSL to TXRX to SLTX to SLRX to TXTX to RX TR CC CC

Ruzzelli, Cotan O’Hare, Tynan, Havinga Phase 2: Switching energy assessment The values obtained are applied to the SMAC protocol; The choice of SMAC has been dictated by the fact that is normally used as benchmark against other novel architectures; Results obtained by using the OmNet++ simulator

Ruzzelli, Cotan O’Hare, Tynan, Havinga The SMAC protocol SMAC divides time in two periods: active time and sleeping time; Active period is divided into three contention based access sections: the SYNC period for node synchronization update, the Request To Send (RTS) period and the Clear to Send (CTS) period. For establishing a communication, neighboring nodes synchronize to the start of the active period by local broadcast of SYNC packets. the exchange of data messages follows the RTS/CTS/DATA/ACK handshake mechanism; therefore, nodes switch between different states periodically. RTS CTS Data Transmitter Receiver time ACK

Ruzzelli, Cotan O’Hare, Tynan, Havinga Simulation setup Three nodes and one gateway arranged in a line, node transmitting range allows nodes communicating with direct neighbours only. Node 3 = Source; Node1 & Node2 = Forwarder; Gateway = Destination Results averaged between node2 and node1 values (node1 and node2 are subject to more switching activity due to message forwarding) 13 independent simulations of 20 minutes each. Clock skew and offset inaccuracies obtained by randomly choosing among 10 independent seeds for each simulation. Traffic load regulated by Node 3: one message of 16 bytes every 60s (low traffic) and 2s (high traffic).

Ruzzelli, Cotan O’Hare, Tynan, Havinga Performance evaluation metrics Energy TX %: energy spent by one node for every bit transmitted; Energy Switch %: energy spent per node for the total number of transitions of two consecutive states; Energy Sleep %: energy spent by one node during the time of inactivity referred to as the sleeping state; Total energy consumption: includes all previous metrics together with the energy spent for receiving and idle listening. It is calculated per node Duty cycle could be changed by varying the node active period in SMAC

Ruzzelli, Cotan O’Hare, Tynan, Havinga Simulated results (1): Total consumption Low traffic High traffic The simulations ended after 50 packets were correctly relayed from source to destination; The results show only a little increase of consumption in high data traffic conditions; The CC family present higher energy consumption profile than Tr1001 due to: –The processor built; –The oscillator left ON in low power mode (oscillator OFF → >5mA current consumption to wake-up)

Ruzzelli, Cotan O’Hare, Tynan, Havinga Simulated results (2): Low traffic condition For all transceivers and duty cycles, switching energy is between the sleeping energy and energy TX; Switching energy can be higher than the energy TX. Lower bound of 1.7% for the duty cycle due to an intrinsic operational limit of SMAC. Other existing protocols that can work below 1% duty cycle (e.g. BMAC) Switching energy as percentage of the total consumption

Ruzzelli, Cotan O’Hare, Tynan, Havinga Simulated results (3): High traffic condition Maximum switching value above 6% for 1.7% duty; Oscillator ON causes higher sleeping energy of CC family thanTR1001. Expected higher % of switching energy for duty cycle lower than 1.7% Switching energy as percentage of the total consumption

Ruzzelli, Cotan O’Hare, Tynan, Havinga Considerations and guidelines Considering 5% as the lower bound of energy consumption significance: –For TR1001 and CC1010, the switching energy needs to be computed if the node duty cycle is equal to or less than 3% and 3.6% respectively; –For CC1000, the switching energy needs to be computed if the node duty cycle is equal to or less than 2.7% and 3.6% respectively; –Sleeping energy consumption of TR1001 can be neglected in any case simulated as less than 2%; –For CC1000 and CC1010 in low traffic load conditions, the transmitting energy becomes significant at 2.5% duty cycle or lower. Although similar, total energy consumptions might greatly differ in their inner energy usage composition  The choice of a protocol to use is not only based on the application but also on the radio on board

Ruzzelli, Cotan O’Hare, Tynan, Havinga Conclusion Values of switching energy have been obtained by direct measurements on different boards; The measurements have been applied to the SMAC protocol; Considerations and protocol assessment guidelines have been derived; In low duty cycle sensor-nets, the switching energy should be computed together with transmitting, receiving and sleeping energies; The obtained results help improve the MAC protocol evaluation process and empowers decisions relating to the judicious protocol/hardware choice for an specific set of WSN applications; Switching energy is expected to account for an even more significant percentage of the total power consumed as the duty cycle get closer to 1% such as in BMAC; Future work activities include the investigation of TDMA protocols that allow lower node duty cycle and more complex topologies.

Ruzzelli, Cotan O’Hare, Tynan, Havinga Thank you for your kind attention! Questions are welcome! Thank you